A molded case circuit breaker having a thermal magnetic trip unit wherein the armature in the form of a flat metal plate arranged proximate the upper ends of a U-shaped magnet piece is described in U.S. patent application Ser. No. 817,213, filed Jan. 8, 1986, and entitled "Interchangable Mechanism For Molded Case Circuit Breakers". The U-shaped magnet piece described therein is arranged around a part of the bimetal for magnetic interaction with a movable planar armature when the current through the breaker exceeds a threshold value. The Patent Application is incorporated herein for reference purposes and should be reviewed for a good description of the interaction between the trip unit and the operating mechanism to trip the breaker under predetermined conditions of overload. The bimetal part of the trip unit responds to overcurrent conditions of a first order of magnitude which is allowed to persist for a prescribed period of time before the operating mechanism is articulated to open the contacts. The magnetic trip unit responds to overcurrent conditions such as a short circuit which is several orders of magnitude higher. In some applications, it is desirable to trip the breaker when an overload condition occurs less than short circuit overload conditions in a shorter period of time than that provided by the bimetal. One purpose of the instant invention is to provide a thermal magnetic trip unit wherein the trip thresholds for the magnetic trip unit are reduced by means of an improved magnetic attraction between the armature and the magnet piece therein.